The quest for the extension of silicon from the base material for electronics towards the base material for photonics spans the development of a small number of key building-block components. One such component is a waveguide which can transmit near-infrared radiation among the different devices of a photonic circuit. A practical implementation can be realized by Si/SiGe/Si heteroepitaxial structures. For these guides, uniformity of the refractive index is an extremely important parameter, since even small local variations can result in large differences in transmission properties.

We present a work where images of small local variations of strain in planar waveguides are acquired by Raman spectroscopy, the only characterization technique which combines strain sensitivity with sub-micron spatial resolution. The strain modulation is of the order of 0.02% with a length scale of the order of several microns and explains a modulation of the refractive index observed in infrared transmission images. The correlation of Raman results with finite-element models show that the physical origin of the modulation is consistent with bunching of dislocations.

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